[Rd] C/C++ 'assert' should not be used in R packages

Duncan Murdoch murdoch at stats.uwo.ca
Sat Nov 10 12:38:17 CET 2007

Prof Brian Ripley wrote:
> Please don't use 'assert' in R packages.  If called, this means that an 
> error in your code aborts the whole R process, including your user's work. 
> I see several R packages doing this, and one of them called 'assert' on me 
> earlier in the week.
I partly disagree about this.  If assert() is triggered, it clearly 
indicates a bug in the package.  If it just generated an R error, most 
users would ignore it, and not report it to the package maintainer. 

It may well be that when an assertion fails, none of the subsequent 
calculations are reliable, in which case returning control to the user 
could result in data corruption.  That's worse than losing a session, 
because at least when you lose a session, you know it.

Could we write our own implementation of assert() that displays an R 
error and unloads the package?  I think I could do something like that 
in Windows by calling FreeLibrary to unload the DLL, but I'd prefer a 
cross-platform solution.
> We provide 'error': please do use it to return control to the user when 
> your code misbehaves.
> Similarly 'exit' and 'abort' should never be used in R packages.
> Sometimes it is not under your control: I sometimes see an rgl failure at
> R: indirect_vertex_array.c:659: emit_DrawArrays_old: Assertion 
> `elements_per_request >= count' failed.
> that is coming from the Mesa GL libraries.
I'd say that's a bug, either in Mesa GL or in rgl.  If you can make it 
reproducible, I'll try to track it down.

Duncan Murdoch

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